The invention relates to x-ray diagnostic apparatus utilizing a video camera; at least one image memory; a subtraction stage for producing a difference signal representing the difference between a stored video signal and a video signal which chronologically follows the stored video signal; an evaluation circuit for time evaluation of the difference signal; and a color video monitor for displaying the processed x-ray images. The evaluation circuit in this diagnostic apparatus contains a time stage and a color converter. Subtraction images are used for x-rays, especailly for selective angiography, to make clearly visible blood vessels that are difficult to recognize in a regular x-ray picture or are located behind bone structures.
The U.S. Pat. No. 4,398,213 discloses x-ray diagnostic apparatus in which an evaluation circuit recognizes the chronological occurrence of changes in the differential image and reads the respective pieces of time information received from a time phase into a time memory by means of a comparator. These pieces of time information consist of numerical values, and the magnitude of the stored numerical values characterizes the chronological occurrence of the maximum of the contrast medium flow. These numerical values are converted into video signals of different colors in a color converter coupled to the memory and are subsequently reproduced on the color monitor. The progression of the contrast medium can thus be observed on the monitor with respect to its location and chronological occurrence with the aid of this color image. For example, a blood vessel through which the contrast medium is flowing is shown in color, and the red image dots may signal an early point in time, while the blue image points represent the chronological end of the contrast medium flow. Thus the entire image with its colored time information pieces can be seen upon completion of the x-ray photography. However, misinformation with respect to the image points occurs with the use of a comparator for the detection of maxima in the difference signal, which leads to color disturbances in the color subtraction image due to quantum noise.